We have previously demonstrated that senescent mouse skin, even when growing on a young syngeneic recipient is characterized by A) an increased susceptibility to skin tumor induction with DMBA and beta-irradiation, B) a decreased content of chalone-like material, C) unresponsiveness to the effect of i.p. injected chalone-like material, and D) (in one experiment) development of more leukemias after DMBA painting of the senescent skin than after treatment of younger skin. We now propose to study further virologic, immunologic and chalone mechanisms that could be important for the modifying effect that skin senescence has on skin tumor development after DMBA treatment. Furthermore, to test if senescent skin also has enhanced susceptibility to the skin-tumor-inducing effect of UV-light, if UV-irradiation can induce leukemia and if it can be confirmed that skin senescence enhances the leukemia inducing effect of DMBA treatment of the skin.